The present invention relates to a lateral power transistor.
For a better understanding of the invention explained below, the basic construction of a lateral power transistor will firstly be explained with reference to FIG. 1. The power transistor has a semiconductor layer 101 of a first conduction type arranged on a substrate 102, which may be a semiconductor substrate or an electrically insulating substrate. Said semiconductor layer 101 comprises a source zone 11 and a drain zone 12 arranged at a distance from one another in a lateral direction of the semiconductor layer 101. A section 13 of the semiconductor layer 101 that is adjacent to the drain zone 12 in the direction of the source zone 11 forms a drift zone of the power transistor. A body zone 14 doped complementarily with respect to the source zone 11 and the drift zone 13 is present between the source zone 11 and said drift zone 13. The source zone 11 and the drain zone 12 are of the same conduction type in the case of a power transistor formed as a MOSFET and are doped complementarily with respect to one another in the case of a power transistor formed as an IGBT.
The source zone 11 is contacted by a source electrode 41, which optionally—via a highly doped connection zone—also contacts the body zone 14, and thereby short-circuits the source zone 11 and the body zone 14. The drain zone 12 is contacted by a drain electrode 42.
In order to control an inversion channel in the body zone 14 between the source zone 11 and the drift zone 13, a gate electrode 21 is present, which is insulated from the semiconductor layer 101 by means of a gate dielectric 31. Said gate electrode 21, at a distance from the body zone 14, undergoes transition to a field plate 22, which is insulated from the semiconductor layer 101 by means of a field plate dielectric 32, which is thicker than the gate dielectric 31.
In this case, the field plate 22 is arranged at a first height level h1 relative to the semiconductor layer 101, while the gate electrode 21 is arranged at a height level h2, which is lower in comparison with the first height level h1, relative to the semiconductor layer 101.
The power transistor turns off if a potential difference between a potential of the gate electrode 21 and the source zone 11 is lower than the so-called threshold voltage of the transistor. With the component in the off state, no inversion channel is formed in the body zone 14 between the source zone 11 and the drift zone 13. With voltage present between the source and drain electrodes 41, 42 or the source zone 11 and the drain zone 12, a space charge zone forms in the drift zone 13 proceeding from the pn junction between the body zone 14 and the drift zone 13. Through the space charge zone there is an increase in the electrical potential in the drift zone 13 (in the case of an n-channel MOSFET or an IGBT) proceeding from the body zone 14. This leads to a voltage stress of the gate dielectric 31, said voltage stress being lowest directly in the region of the pn junction and increasing in the direction of the drain zone 12. In this case, the dielectric strength of said gate dielectric 31 critically influences the dielectric strength of the component. The voltage stress of the gate dielectric 31 can be reduced, in order thereby to increase the dielectric strength of the component, by making that section of the gate electrode which overlaps the drift zone 13 as short as possible. However, a small overlap between the gate electrode 21 and the drift zone increases the on resistance of the component when the latter is in the switched-on state. In other words: a larger overlap between the gate electrode 21 and the drift zone 13 reduces the electrical resistance in the transition region in which charge carriers pass into the drift zone 13 from the accumulation layer in the drift zone 13 below the gate electrode 21.
The voltage stress of the gate dielectric 31 thus increases the more the gate electrode 21 overlaps the drift zone 13 in the direction of the drain zone 12; in this case, voltage spikes or field strength spikes of the electrical occur in particular in the region in which the gate electrode 21 undergoes transition to the field plate 22 or in which the thinner gate dielectric 31 undergoes transition to the thicker field plate dielectric 32.